Overload control system for transistor amplifiers



Aug. 27, 1963 N. J. JOHNSTONE OVERLOAD CONTROL SYSTEM FOR TRANSISTORAMPLIFIERS Original Filed May 27, 1957 3 Sheets-Sheet 1 c c COMPARATORCOLLECTOR VOLTAGE-e 3 lzwmmnu @2838 JMTNT INVENTOR.

NORMAN J. JOHNSTONE ATTORNE Aug. 27, 1963 N. J. JOHNSTONE 3,102,241

OVERLOAD CONTROL SYSTEM FOR TRANSISTOR AMPLIFIERS Original Filed May 27,1957 :s Sheets-Sheet 2 In 5o -sov.

l. INVENTOR. 30 I; NORMAN J. JOHNSTONE Aug. 27, 1963 N. J. JOHNSTONE3,102,241

OVERLOAD CONTROL SYSTEM FOR TRANSISTOR AMPLIFIERS Original Filed May 27,1957 5 Sheets-Sheet 3 SIGNAL WAVE FORMS AT VARIOUS LOCATIONS IN THECONTROL SYSTEM NORMAL ABNORMAL (SAY SHORT CIRCUIT) LOAD LOAD INVENTOR.

NORMAN J. JOHNSTONE ATTORNEY I and E.

This application is a continuation of the copending application filedMay 27, 1957, Serial No. 661,777, entitled Overload Control System forAmplifiers, and assigned to the assignee of .this application, and nowabandoned.

This invention relates to overload protection systems for amplifiers.Many amplifiers operating at or near rated load are quite vulnerable toburn-out caused by overload, momentary short circuits in the loadcircuit of transistors often causing complete destruction within oneexcursion of the signal voltage. The .usual tuse, mechanical circuitbreaker, or the like, is too slow [to protect such amplifiers. The usualautomatic gain .control circuits, comprising integrating means toraveraging signal levels, also are too slow to protect transistor-typeamplifiers. The heat dissipatingcapaci-ty of the junction-typetransistor is particularly small and any protecting circuit mustfunction instantaneously. Further,

United States Patent F r the collector voltage ordinate.

overload protecting systems usually respond to abnormal current, I,through'the loader 'to abnormal voltage, E, across the load rather thanto the combined effects of An object of this invention is to provide animproved overload protecting circuit for amplifiers.

A more specific object of this invention is to provide a protectingcircuit WhlChdS responsive to the combined effects of current throughand voltages across the amplifier to be protected.

A still more specific object of this invention is to provide an improvedprotecting circuit for burn-out vulnerable transistors.

A. still more specific object of this invention is to prov-ide atransistor protecting circuit which functions instantaneously uponoverload. a

The objects of this invention are attained by sampling the signalcurrent through the amplifier and the signal voltage across the outputcircuit of the amplifier to derive a cyclic voltage'which is a functionof the-combined values of the sampled current and voltage. In oneembodiment, the combined values are analogous to the power dissipated inthe amplifier and is employed to regulate the signal driving voltage.important feature of the invention, the protecting circuits respondinstantaneously to cyclic signal voltage to protect the transistor-typeamplifier having low heat dissipating capacity. V

Other objects and featuresv of the invention will become apparent tothose skilled in the art by referring to the tollowing specification ofpreferred embodiments, shown in the accompanying drawings, in which:

"FIG. 1 is a blocked circuit diagram of one amplifier embodying thisinvention;

FIG. 2 shows the load characteristics of the amplifier oil- 16.1; 1

FIG. 3 is a detailed circuit diagram of a practical operative amplifierembodying this invention; and

FIG. 4 shows a family of voltage waves at strategic points in the systemof FIG. 3.

The amplifier to be protected, shown in FIG. 1, is the transistor .1,with base 2, collector 3, and emitter 4. The particular connectionsshown are of the commonemitter type, with the base and emitter connectedacross According to an 3,102,241 Patented Aug. 27, 1963 the secondary of.a signal coupling transformer 5, and

with the output electrodes, including the collector'and emitter,connected across the primaryof the output trans- The driver foramplifier '1 may, if desired, comprise another transistor 7, the gain ofwhich may be regulated by the biasing resistor 8 in the emitter circuitthereof. A grid controlled thermionic amplifier could be substituted forthe driver 7.

In FIG. 2 is. shown the load characteristics of a typical transistoramplifier. By plotting collector voltage, e against collector current, ia family of curves may be obtained .for various base biasing currents.The usual load line AB is obtained by plotting points A and B,respectively, for maximum and minimum collector currents encountered inoperation with normal load. The normal instantaneous dissipation ofpower in the amplitier is the-shaded rectangular areadefined by voltagea, and collect-or current t subscript X denoting any point along theload line AB. In normal operation, point X moves back and forth alongline AB at signal frequency. If, however, the load on the transistor :1is increased, as by a decrease in impedance Z which in the limitcondition is shortcircuited, the load line AB is shifted to line A B,substantially perpendicular to It will be noted now that with ashortcircuited load condition the maximum energy to be dissipated in theamplifier is substantially four times normal maximum dissipation. Ii:the ampli-v fier is a commercial junction transistor, there is littleheat-sinkf capacity and the transistor can be ins-tanta" neous-lydestroyed by the overload.

Accordingly, it is an important feature of this in-' vention to providemeans for sensing abnormal power. dissipated in the amplifier. That is,signal -current,I, through the amplifier and signal voltage, E, acrossthe amplifier are sampled and a voltage proportional to a tunction ofthe instantaneous combined values of I and E is fed back to the driver.As will appear, the objects of this invention are attained by circuitswherein information tted back to the driver for protecting the amplifieris representative oi the instantaneous product, I E, or theinstantaneous sum, 1+E, of the two sampled cyclic voltages.

The collector, or output, current is convenientlysampled according tothis invention by inserting the resistor 9 in the output circuit. Whilethe resistor may be located any place in the output circuit, it ispreferred that the resistor be connected immediately adjacent to theemitter. The voltage drop across resistor 9 is proportional to theoutput signal current and is analogous to i,.,. The ohmic value of theresistor 9 need be but a negligible proportion of the total seriesresistance of the emitter current, i

' circuit and should have ample current carrying capacity.

conveniently derived between the collector and ground by lead 10. Lead10, accordingly, samples the collector voltage, e and the lead 11directly samples the collector The two voltages are combined in thecomparator 12 to produce at its output 13 a voltage proportional, in theparticular embodiment shown in FIG. 1, to e i For example, the variablegain multigrid vacuum tube multiplier disclosed on page 253 ofElectron-ic Analog Computers by Korn and Korn, 2d edition,-McGraw-Hill,1956, may be used, if desired.

The product voltage at the output of the multiplier is fed back to thedriver 7 via line 13. The feedback information on line 13 is preferablyamplified, asshown in FIG. 1, by connecting line 13 to the base oftransistor 14. Conveniently, the biasing resistor 8 for the emittercircuit of driver 7 is included in the emitter circuit of amplifier 14and is so proportioned that the negative feedback signal on the line 13and base of transistor 14 can properly control the gain of driver 7should the power dissipated in transistor 1 exceed a predeterminedamount. It may be found necessary to bypass the biasing resistor with aconventional bypass condenser 8a to prevent excessive attenuation of theA.C. signal voltage applied to the input of driver 7. It is contemplatedthat the parameters of the feedback circuits be so chosen that when loadZ is short-circuited, the collector current of the protected amplifierbe limited to some low value such as i FIG. 2, for full collectorvoltage. It is further contemplated that the time constants of themultiplier and feedback circuits be sufiiciently low that the voltageindicating overload can instantly be applied to the driver 7.

During normal operation, the transistor 14 is cut 01f by a slightlypositive base with respect to the emitter, so that the emitter currentis negligible. During abnormal operation, as when the load on transistor1 is short circuited, the transistor 14 becomes conducting by anegative-going signal pulse and the emitter current becomes suddenlyrelatively high. With the P-N-P type transistor as shown, the negativesignal voltage on line 13 is converted to a low impedance source and iseffectively connected to the emitters of both transistors 14 and 7through the low resistance of transistor 14 the instant the base voltagegoes negative in response to an alarm signal on line 13. The negativepotential on the emitter of transistor 7 cuts off that transistor andimmediately interrupts the application of signal voltages to thetransistor 1.

In FIG. 3, the amplifier to be protected is a push-pull Class Barrangement comprising transistors 1a and 1b connected in balancedrelation between output transformer 61: and input transformer a. FIG. 3is characterized also by means for obtaining a feedback voltage which isa function of the two sampled cyclic signal voltages. In FIG. 3, theinstantaneous voltage fed back to the driver 7 is derived by adding thetwo sample voltages. Inasmuch as it is desired to only sense dangerousoverloads on the transistor to be protected, it is actually unnecessaryto derive the product of the sampled voltages throughout their normaloperating range. Instead, it is merely necessary to have a feedbackcircuit so constructed that if the product exceeds a certain value thenthe protecting circuit will sense this value. Since the transistor is aconstant current device, the collector current is essentiallyindependent of the collector voltage. Therefore, the collector currentand, hence, the voltage E FIG. 3, is essentially constant. For practicalpurposes then, the dangerous overload condition can be sensed byadditively comparing E with E As will appear, the addition of these twovoltages simply and reliably derives a voltage which will instantly cutoff the driver 7 when an abnormal load condition arises.

Specifically, current sampling resistor 9a and complementing resistor 9bare connected in the emitter circuits in FIG. 3. The voltagesproportional to collector voltage and collector current are obtained,respectively, at the collector and at the emitter of 1b, as in the caseof FIG. 1.

The voltage E at the collector junction includes the DC voltage of thebattery shown as well as the signal voltage. The phase of the signalvoltage at E is opposite to the phase of the signal voltage at the base,as usual. The base and emitter voltages are, of course, in phasealthough the emitter is half-wave rectified. The collector voltage isapplied to cascaded transistors 21 and 22 through lead and the half-waverectifier 20. After amplification by transistors 21 and 22, the signalvoltage is obtained across the emitter resistor 23.

Now, the voltage E produced by transistor current through resistor 9a isapplied through lead 11 to cascaded amplifiers 33, 24, and 25 and isimpressed across th emitter resistor 26. Amplification in the twoparallel channels is adjusted and the parameters of the circuits soselected that the signal voltage drop across resistors 23 and 26 areequal and opposite in normal operation. That is, as point X in FIG. 2moves along load line A-B with normal load Z the electrical center ofresistors 2326 remains fixed. Accordingly, the center of resistor 27,which is in parallel with resistors 23 and 26 as shown, does not changein normal operation of amplifier 1a--1b, and there is no feedback overline 13. Where the transistors 22 and 2-5 are in series and are of thejunction germanium P-N-P and N-P-N type, respectively, the collectorsare operated at a negative and positive voltage, respectively, withrespect to the emitters as shown. In the specific example shown,accordingly, the center point of potentiometer 27 stands at some stablelevel between ground and the negative collector voltage of 22. In theexample shown, with the collector voltages indicated, the mid point ofpotentiometer 27 normally stands at 25 volts.

tAn intermediate point on potentiometer 27 is coupled back to theemitter of driver 7. It has been found desirable, in the particularembodiment of FIG. 3, to amplify the feedback current in two cascadedamplifiers 14 and 32. Amplifiers 14 and 32 are also of the P-N-P typewith the collectors connected to a higher negative voltage than theemitters, as shown. To prevent signal feedback and self-sustainedoscillations or ringing in the system of FIG. 3, the bypass condenser 30in conjunction with the reverse resistance of diode 31 forms aneffective filter to eliminate the possibility of positive feedback,should E become larger in amplitude than E for some reason, such as, anopen circuited load. Obviously, positive feedback could causeoscillations and ringing which might damage the transistor which is tobe protected. It is desirable also to so back bias the rectifier 31 asto prevent negative-going signals below a predetermined amplitude frombeing fed back to the driver. That is, a substantial negative-goingsignal at E which occurs only when abnormal or high-load operatingconditions occur in the output of amplifiers 1a and 1b, is required toovercome the back bias of rectifier 31. In the embodiment of FIG. 3 withthe transistor voltages indicated, the cathode electrode of rectifier 31is biased by line 13 to 25 volts, while the anode electrode is biased tosome voltage between 25 and 5() volts. If the anode bias is, say, -37 /2volts, the back bias is then 12 volts and the E voltage must swing to 37/2 volts before current can start through the rectifier 31. When currentdoes start through the rectifier 31, the resistance of rectifier 31drops from some high or near infinite value to near zero value. Thenegative-going signal passed by the rectifier immediately drives thebase of transistor 32 in a negative direction which in turn unblocks orincreases the emitter current of transistor 32. The resulting negativesignal on the base of transistor 14 increases the emitter current oftransistor 14, increases the voltage drop through resistor 8, and hencereduces the emitter current of transistor-driver 7. The voltage dropthrough resistor 8 can be adjusted, by suitable selection of circuitparameters, to cut oif driver 7 when the first minimum signal passesrectifier 31.

In operation, when the load impedance Z, decreases, the peak-to-peakA.C. voltage across the transistor 1b decreases. Assume load impedance Zdrops to zero, as by a short circuit. This means there can be no signalvoltage developed across the primary of transformer 6a. If there is nosignal voltage developed across the primary, only the voltage of thebiasing battery, which is 25 volts in the example of FIG. 3, is appliedacross the transistor 1b during the time the short circuit is present.This means that no signal voltage E can be applied to the base oftransistor 21 nor transistor 22. However, the signal voltage E isapplied to the base of transistor 33 because the signal in the base andon the emitter of 1a and 1b is still when the new voltage in thefeedback line 13 is sutficient,

there is an increased instantaneous bias current through resistor 8cutting the gain of driver7 to Zero or some new safe value. When theoverload at Z is removed, the driver operation returns to normal.

The family of curves of FIG. 4 have been added to show cyclic signalwaveforms at strategic points throughout the system of FIG. 3 for bothnormal and abnormal load at Z The voltage values applied to thewaveforms,

of FIG. 4 [are the approximate signal voltages when the battery voltagesof FIG. 3 are employed. The full-Wave signal voltage E on the base of 1bis inverted in phase on the collector of that transistor. voltage E isfull wave as shown by virtue of the pushpull operation and the mutualcoupling between the windings on either side of the center tap oftransformer 6a. The voltages E and E on the emitter and on the collectorare phase opposed as shown in FIG. 4. The positive loops of the signalwave at the emitter are eliminated as shown because as the base of 1bgoes positive the transistor is outo-tI making it impossible for theemitter'to follow. The negative loops of the signal wave of B areeliminated by The collector signal rectifier 20 to produce the half-waveof E.,, as shown, so

that it may be adjusted in amplitude and compared to E Voltages E :and Ewhich are amplified undistorted replicas of signal voltage E are inphase inasmuch as transistor 29 is of the N-P-N' type as distinguishedfrom the P-N-P type employed elsewhere in the system. Now, since E andE; at opposite ends of potentiometer 27 are opposed and are adjusted tobe equal, the voltage E remains steady. It is significant that thehalf-wave signal voltages E and E; at opposite ends of potentiometer 27are substantially undistorted replicas of the original signalsE and Eadjacent in amplitude, so that any instant of time their sum is zero. Y

Now, when the abnormal load is applied, the signal component at thecollector of transistor 1b disappears and there remains only the steadyreference voltage E Thereupon E becomes stetady, and E becomes steady.In the meantime, voltages E E and E follow the pattern of the signalwave. The steady voltage E added to the voltage E results in thenegative-going feedback voltage E in line 13, and the protective actionin the I biasing resistor 8-.

It now becomes apparent that the type of amplifiers to be used in thesystem of this invention are not limited to the specific amplifiersillustrated. Further, the sampling circuits generally designated at 10,11, and 12, in FIGS. 1 and 3, may assume many configurations withoutdeparting from the scope of this invention.

What is claimed is:

1. In an overload protecting system for transistors, a

transistor to be protected, an amplifier for cyclic signal,

voltages driving said transistor, a signal voltage sampling circuitconnected across the output electrodes of said transistor forobtainingla sample of the cyclic signal voltage, the sampled signalvoltage being proportional to the impedance drop through the loadcircuit of said transistor caused by transistor output signal current, asignal current sampling circuit in series with said output electrodesfor producing a signal proportional to the signal current through saidtransistor, means for instantby-instant comparing the sampled cyclicvoltages and for producing an output voltage only when the comparedvoltages are, dissimilar and means responsive to said output voltage ofthe comparing means for attenuating the signal driving voltage appliedto said transistor.

2. In combination, a transistor signal amplifier including a transistorwith an input circuit and an output circuit coupled, respectively, to adriving circuit and to a load circuit, said driving circuit having acyclic signal voltage source and a gain control circuit, a resistor ofrelatively small ohmic value in said output circuit; terminals connectedto two output electrodes of said transistor, separate means forseparately amplifying the cyclic signal voltages at each of saidterminals without integration, a potentiometen'means for applying theamplified signal voltages, respectively, to the terminals of saidpotentiometer for continuously algebraically adding the instantaneouscyclic values'of the two amplified signal voltages, and a couplingcircuit connected between an intermediate point on said potentiometerand said gain control circuit of said driver for applying to said gaincontrol circuit a cyclic signal voltage theinstantaneous amplitude ofwhich is a function of the sum of the two cyclic signal voltages addedin said potentiometer.

3. In combination, a transistor amplifier connectedbetween a loadcircuit and a driver circuit, said driver circuit having a biasingcircuit, m-ean-s for respectively sampling the signal current throughand the signal voltage across said transistor, means for comparing theinstantaneous values of said two signals, and a feedback circuit betweenthe comparing means and the bias circuit of said driver for applying theinstantaneous values of combined signal voltages to said biasingcircuit.

4. The feedback circuit recited in claim 3 comprising a rectifierserially connected in said feedback circuit to permit application ofinstantaneous signal voltages of only one polarity to said biasingcircuit, and a bypass condenser connected [across said feedback circuitbetween said rectifier and said biasing circuit for preventingoscillation-pr oducing feedback.

5. In combination, a transistor amplifier having output electrodescoupledin a load circuit and having con trol electrodes coupled to analternating current driver vcircuit, said driver circuit having abiasing circuit; means coupled tothe output electrodes for,respectively, sampling as signal voltages the alternating currentthrough and the alternating voltage across said transistor, means forseparately amplifying said two alternating signal voltages, means foralgebraically adding the instantaneous cyclic phase-opposed values ofsaid two signal voltages, and a feedback circuitc'oupled between theadding means and the biasing circuit of said driver for applying theinstantaneous cyclic values of added signal voltages to said biasingcircuit.

6. The invention as set forth in claim 5 wherein a rectifier is seriallyconnected in said feedback circuit to permit application ofinstantaneous cyclic signal voltages of only one polarity to saidbiasing circuit, and a bypass condenser is connected across saidfeedback circuit between said rectifier and said biasing circuit.

7.In an overload protecting system for transistors, a transistor to beprotected, said transistor having input electrodes and outputelectrodes, a signal amplifier for driving said transistor with a cyclicsignal voltage, a signal voltage sampling circuit coupled to the outputelectrodes the phase-opposed signal voltages at the end terminals sothat the mid-tap voltage remains substantially fixed during normaloperation of said transistor, and coupling means between saidmid-tap andsaid signal amplifier for instantaneously reducing the amplitude ofsignals applied to said transistor during abnormal operation of saidtransistor.

8. In an overload protecting system for transistors, a transistor to beprotected, said transistor having input electrodes and outputelectrodes, a cyclic signal amplifier driving said transistor, a voltagesampling circuit coupled across the output electrodes of said transistorfor producing a cyclic signal voltage proportional in amplitude to thedrop through the load circuit of said transistor caused by transistoroutput signal current; a signal current sampling circuit, directly inseries with said output electrodes, for producing :a cyclic voltageproportional to signal current through said transistor; means forcombining in phase opposition the instantaneous values of said cyclicvoltages to produce a cyclic signal, and coupling means between saidcombining means and said signal amplifier for instantaneously reducingthe amplitude of signals applied to said transistor during abnormalexcursions of signal current through said transistor.

9. In combination, .a transistor signal amplifier with an input circuitand an output circuit coupled, respectively, to a driving circuit and toa load circuit, said driving circuit having a cyclic signal voltagesource and a gain control circuit, a resistor in said output circuit,said resistor having small ohmic value compared to the normal ohmicresistance of said output circuit; terminals connected to the two outputelectrodes of said transistor, separate means for amplifying the twosignal voltages at said terminals, a potentiometer, means for applyingthe two amplified signal voltages, respectively, to opposite ends ofsaid potentiometer for continuously algebraically adding theinstantaneous values of the two amplified cyclic signal voltages, and acoupling between an intermediate point on said potentiometer and saidgain control circuit of said driver to apply to said gain controlcircuit a signal voltage the instantaneous amplitude of which is afunction of the sum of the two signal voltages added in saidpotentiometer.

10. In combination, a transistor with base, emitter, and collectorelectrodes coupled as an amplifier between an output circuit and asignal driver circuit, said base and emitter electrodes being connectedin said driver circuit,

and said collector and emitter electrodes being connected in said outputcircuit, a resistor of small ohmic value compared to the resistance ofsaid output circuit connected to said emitter in said output circuit,means connected directly to the emitter-end of said resistor forsampling the signal voltage across said resistor, means connecteddirectly to said collector for sampling the signal voltage across saidoutput circuit; an adding resistor, coupling means for applying in phaseopposition said two sampled signal voltages, respectively, to oppositeends of said adding resistor whereby an intermediate point on saidadding resistor is normally of fixed voltage, said point being coupledto said driver circuit to cut-01f said driver in response to abnormalload conditions.

11. In combination, a transistor amplifier with a base, a collector, andan emitter, a driver amplifier with a biasing circuit, said driveramplifier being transformer-coupled to said base and emitter, saidcollector and emitter tbeing transformer-coupled to a load circuit, aresistor in the emitter circuit of said transistor amplifier, an addingresistor, means for applying cyclic signal voltages at said collector inone phase to one end of said adding resister and means for applying thecyclic signal voltage at said emitter in opposite phase to the other endof said adding resistor for combining the two mentioned signal voltages;and feedback circuit, responsive to the combined voltages at anintermediate point on said resistor, coupled to-said biasing circuit ofthe driver amplifier.

References Cited in the file of this patent UNITED STATES PATENTS2,504,699 Kluender Apr. 18, 1950 2,561,049 Buys July 17, 1951 2,672,530Ensink Mar. 15, 1954 2,760,007 Lozier Aug. 21, 1956 FOREIGN PATENTS789,582 Great Britain Jan. 22, 1958

3. IN COMBINATION, A TRANSISTOR AMPLIFIER CONNECTED BETWEEN A LOADCIRCUIT AND A DRIVER CIRCUIT, SAID DRIVER CIRCUIT HAVING A BIASINGCIRCUIT, MEANS FOR RESPECTIVELY SAMPLING THE SIGNAL CURRENT THROUGH ANDTHE SIGNAL VOLTAGE ACROSS SAID TRANSISTOR, MEANS FOR COMPARING THEINSTANTANEOUS VALUES OF SAID TWO SIGNALS, AND A FEEDBACK CIRCUIT BETWEENTHE COMPARING MEANS AND THE BIAS CIRCUIT OF SAID DRIVER FOR APPLYING THEINSTANTANEOUS VALUES OF COMBINED SIGNAL TO SAID BIASING CIRCUIT.